The use and development of laboratory fume hoods for cutting edge research dealing with everything from bioterrorism to the human genome has resulted in many inventions to handle harmful materials safely and engendered much debate about the best way to control airflow through the fume hood. This debate concerns the capture of contaminants and the prevention of their escape into the surrounding environs where the lives of laboratory researchers, students, teachers, occupants, technicians and other personnel may be threatened. Various types of fume hoods with various types of configurations all utilize various sash mechanisms which promise safety to the user by their closing the sash during their experiments so that an exhaust fan can draw toxic fumes, pathogens and contaminants inside the hood away from the operator, and exhaust them through a laboratory exhaust ventilation fan. Dangers of contamination exist, however, with respect to periods of time when the sashes of a respective fume hood are left open and there is much debate over the minimum face velocities that must be maintained for the fume hood user to be kept “safe”. Further, there is also much debate with respect to what types of sensing mechanisms should be used to keep a user safe such as airflow measurement or sash sensing or a combination of both.
Various sash sensing devices have evolved to provide the quick speed of response necessary to maintain safety while also providing easy maintenance based on adjusting the blower and thus the exhaust volume of the hood linearly in proportion to the change in opening size of the hood to maintain a constant face velocity. This principle is for conventional fume hoods that form an enclosure that uses a horizontal or vertical sash which slides horizontally and/or vertically to provide a variable opening. The amount of air exhausted by the hood blower is constant, and the face velocity increases as the area of the sash opening decreases. See, U.S. Pat. Nos. 4,528,898 and 4,706,553. These systems calculate an assumed face velocity based on the position of the sash when the system is set up. Exhaust measured in cubic feet per minute (CFM) in the duct is measured and corresponding sash positions are assumed to result in particular face velocities based on the opening in the fume hood. These systems provide the advantage of quick response to changed airflows around the fume hood. That is, as sashes are raised and lowered, a mechanical linkage to the venturi valve, blade damper or other device is also moved proportionally to ensure that the corresponding CFM necessary to maintain an assumed face velocity based on the position of the sash at the hood will result. As sashes are moved up and down, the system thus, responds to adjust airflow accordingly.
U.S. Pat. Nos. 4,893,551 and 5,117,746b discuss additional styles of fume hoods wherein two or more sashes are mounted to slide horizontally on at least two tracks which are located on the top and bottom of the sash opening. They also apply to fume hoods which have sashes mounted on tracks for horizontal movement, which tracks are, in turn, mounted on a sash frame which may be moved vertically; i.e., a combination sash having a combination sash frame. These patents also discuss techniques which may be utilized with such sashes to determine the sash opening. As is noted in these patents, with two or more sashes, absolute position of the sashes is not sufficient information by itself to indicate the open area of the hood. Instead, it is the relative position of the two or more sashes of the hood which determine the total open sash area. The problem becomes even more complex where four sashes are mounted on two tracks, which is a very common configuration, or where the hood is being moved both horizontally and vertically.
In the U.S. Pat. No. 4,893,551, the sash opening detection function is performed, in general, by having a source of radiation, and a detector for such radiation, and by mounting the source and detector relative to each other and to the sashes such that the amount of radiation detected is proportional to the uncovered portion of the opening. For preferred embodiments of the patent, various discrete magnetic or optical emitters and sensors mounted adjacent to or on the sashes are utilized to determine the fume hood opening. So, in this embodiment, sash position sensing uses assemblies of sensor elements mounted to the moveable sashes whose position is desired to be detected. Each assembly of sensor elements is electrically connected to external electronics through a sensor cable. Although this prior art is preferred over other available technology, such electrical connection methods for sash position sensing are less than optimal, particularly for cases where sensing is to be provided for horizontal sash, combination sash, or walk-in hood types. Routing the horizontal sash sensor cable presents difficulties related to either the establishment of operative pivot points or mounting a take-up reel for cable movement. The issues faced include both real and perceived reduced reliability over time due to cable wear, difficulties in installation, and the poor aesthetics of exposed cable that moves in a pendulous manner.
Other issues with conventional technology have been with the thickness of the sensor and magnet bars, given the increasing trends for tighter hood construction and, thus, reduced spacing between sashes from one track to another. Alternatively, a ¾″ limitation on maximum distance between the surface of the sensor bar magnet and that of the reed switch sensor assembly is occasionally an issue with larger, more loosely designed hoods, so improvements in sensor sensitivity is desirable. See U.S. Pat. No. 4,893,551.
Recent developments such as U.S. Pat. No. 6,137,403 show a fume hood sash sensor using multiplexed sensors to measure sash position. The sensor transmitter or receiver elements may be multiplexed. Furthermore, the sensor may employ passive, passive remote powered transponder, or powered transponder elements on the sashes to measure sash position. The multiplicity of elements can be cost prohibitive and difficult to maintain. Also, see U.S. Pat. No. 6,358,137 which uses a rotary position sensor with a lever arm mechanism which translates horizontal or vertical movement to rotary movement for determining the position of the sash door. The apparatus compensates for nonlinearity that results from the translation. However, this invention has proved to be impractical in the field, expensive, and not widely used due to the need for using an awkward lever in tandem with the rotary sensor.
Consequently, what is truly desired is a system that minimizes equipment so that cables, pulleys and wheels are eliminated while providing an easy to install yet effective way to measure the sash as it changes.